3. SPIRAL GALAXIES WITH UNUSUAL FAINT FEATURES

Here we present examples of gas-rich galaxies without obvious interactions
or companions that nonetheless show evidence of interaction. A nearby
example is NGC 253, in the Sculptor group. The extent of the optical halo of
this galaxy is much greater than that of the neutral hydrogen envelope
detected by
Koribalski et al.
(1995),
and it also extends well beyond
the fields where Hawthorn has detected diffuse H-alpha emission
(unpublished). Fig. 4 is the result of combining
the photographically
amplified derivatives of five deep IIIa-J (ie. blue-green) plates taken with
the UK Schmidt Telescope. The image has a limiting surface brightness of
about 28 mag/arcsec-2 and shows that the low surface brightness
envelope is devoid of fine detail and is unusual in not being strongly
truncated, as is the case for most spirals. However, it also shows a
distinct, asymmetrical extension in the southern half of the galaxy. This is
difficult understand, since NGC 253 is not obviously interacting with other
members of the Sculptor group, and no other structures in the disk
of the galaxy, or in the HI velocity profile, suggest any kind of external
disturbance.

Figure 4. The extent of the faint outer
envelope of NGC 253 is evident when compared to the normal contrast
positive image, inset to the same scale. The scale bar is 30 arc min.

Preliminary calculations by Hawthorn (unpublished) show that neither line
emission nor scattered light can account for the faint light in the
400-530nm passband recorded here, so the most likely explanation is that we
are seeing an extended, asymmetrical stellar halo, perhaps distorted by the
infall of a companion galaxy which has long since been absorbed. However, it
should be noted that the southern part of the galaxy, where the anomalous
extension is most obvious, is also the region with the strongest
polarisation at 1.4 GHz, and the 0.33 GHz diagram also shows a southern
spur. Since NGC 253 is seen almost edge-on, the anomalous
extension could be
a line-of-sight loop or arc effectively detached from the main body of the
galaxy, either the result of an ancient encounter, or, perhaps the debris
from the incomplete disruption or absorption of a Magellanic Cloud-like
companion.

M83 (NGC 5236) is a spectacular face-on spiral
galaxy. Colour photographs
show it to have clumpy, well-defined spiral arms, evidently rich in young
blue stars. The galaxy is remarkably symmetrical and has no obvious
companions or evidence of interaction. However, deep images
(Fig. 5) reveal
an extensive but very faint arcuate structure to the north west of the
galaxy, clearly centred on it, but completely disconnected from it. The
arc has a maximum projected distance of about 55 kpc from the nucleus of
M83 and is
of a similar length, though this is very difficult to determine
precisely. The structure was first noted some years ago by Cannon
(private communication). It is apparently devoid of the fine detail
typical of the extensive but faint
Galactic reflection nebulosity which is detectable in the Centaurus-Hydra
direction and is seen on many deep plates taken on different centres, so is
certainly real.

Figure 5. Even well-known galaxies can
spring surprises. This deep image of
M83 reveals an enormous loop around the NW quadrant
of the galaxy. The inset image is the same scale as the main
picture. The scale bar is 30 arc min.

The galaxy also has a very extensive HI halo with 80 percent of the
detected HI beyond the Holmberg radius, much more than is normal in such
galaxies
(Huchtmeier and
Bohnenstengel 1981).
A distinct ridge in the HI velocity profiles in seen in their data, at a
position that corresponds to the optical loop, but extending well beyond
it. If M83 were strongly
inclined to the line of sight, it may well be that the faint loop would
appear as an
extension to one end of the galaxy, rather like that in NGC 253.

The well-known Sombrero galaxy, M104 (NGC 4594) shows no signs of
interaction even on the deepest optical images, nor is there anything
unusual
in the HI distribution or velocity structure, and though the HI mass is
unusually small, there is no evidence of large radial velocities
(Bajaja et al. 1984).
However, M104 shows an unexpected faint feature at a
considerable projected distance to the SSW of the galaxy (position angle
about 200 degrees). This takes the form of a faint, loop
(Fig. 6) which appears to be associated with M104, but, like that in M83, seemingly detached from it. The loop is at a
projected distance of about 20 arc min, extending 100 kpc
from the nucleus if the distance to M104 is taken as 18.6 Mpc. The loop is
sharp-edged and quite distinctive, but in the image reproduced as
Fig. 6
there is also a strong suggestion of a more diffuse faint structure of
similar extent to the NNE of the galaxy, diametrically opposed to the
loop at position angle 20 degrees.

Figure 6.M104 looks like an E2 galaxy at
low surface brightness. The outer
`elliptical' component is featureless except for a faint loop to the SSW
and a corresponding faint feature without structure to the NNE. Scale
bar is 30 arc min.

There is no previous mention of this feature in the literature, and
there is no evidence of these structures in the contours of M104 reproduced by
Burkhead (1979).
However, Burkhead's contours, which extend to 28
mag/arcsec-2, were red-light exposures aimed at tracing the
extent of the red bulge of M104, while the image reproduced in
Fig. 6 was made on
IIIa-J plates which would tend to emphasise blue features.

Our final example of a spiral galaxy with unexpected faint features is
NGC 2855, an SA(r) galaxy remarkable for its
ordinariness. It appears in
many lists of galaxies in the literature but is not singled out as in
any way notable.
However, combined multiple exposures reveal an enormous, faint, almost
complete loop or perhaps a disk seen in projection. Like most of the faint
features noted here, it has a surface brightness fainter than 28
mag/arcsec-2. At this stage, that is all that can be said
about it, but the enormous extent of this feature makes NGC 2855 worthy of further study.

Figure 7.NGC 2855 seems to be a perfectly
ordinary SA(r) galaxy that has not
previously attracted attention. However, its very extensive low surface
brightness disk or ring suggest that the dynamics of this galaxy are
unusual.